Practical methods for introducing foreign DNA into bacteria and other organisms are, of course, known in the art. These methods employ recombinant DNA technology to construct chimeric plasmid DNA molecules which contain pieces of foreign DNA. The chimeric plasmids are designed to replicate autonomously as an extrachromosomal DNA species in a procaryotic host such as E. coli. In yeast, the chimeric plasmid may either replicate extrachromosomally or it may integrate into yeast chromosomes, although such integration tends to be unstable. When employing such genetically engineered microorganisms to manufacture products, it is important to continously select for the presence of the foreign DNA in the recipient organism because the foreign DNA carried on a plasmid or in integrated form tends to be unstable and can be lost in daughter cells.
Many applications of genetic engineering technology are centered around the production of proteins via genetically altered microorganisms (e.g., insulin or interferon). Production of a protein often requires only that a single gene encoding the specific protein be introduced into the recipient organism. The synthesis of more complex metobolic products, however, may require the introduction of several foreign DNA segments or genes to create new biochemical pathways. For example, the production of nonprotein products could be achieved if genes encoding the production of enzymes which operate on nonprotein substrates to produce the desired product could be stably introduced into a microorganism. It is difficult to maintain a multistep biochemical pathway using "traditional" technology because of the instability of the foreign DNA introduced via the chimeric plasmid DNA molecules. Simply too many genes must be introduced and maintained collectively in the recipient organism. A means for introducing several foreign DNA segments or genes stably in a recipient organism would, therefore, be a valuable contribution to the art.